1. Field of the Invention
This invention relates to sliding members having superior wear resistance, toughness and adhesion, and is effective when applied to sliding members such as piston rings used in internal combustion engines.
2. Description of the Related Art
In recent years, piston rings have been subject to the increasingly harsh operating environments due to higher engine outputs and changes to meet exhaust emission regulations. Many engines which were processed with conventional surface treatments such as hard chromium plating and nitriding for surface processing of sliding surfaces, are not durable enough to withstand this harsh environment.
To respond to this problem, piston rings covered with a film of titanium nitride or chromium nitride which can be thickened using physical vapor deposition have been proposed (see Japanese Patent Laid-open No. 57-57867). Further, piston rings covered with a film of molybdenum nitride having wear resistance superior to chromium nitride have been proposed (Japanese Patent Laid-open No. 2-134468, Japanese Patent Laid-open No. 6-17933, and Japanese Patent Laid-open No. 6-25826), in order to extend the piston ring operating life.
Thermal sprayed films are known which contain hard particles consisting of carbide, nitride or oxide dispersed within molybdenum metal. However these films have a composite structure containing hard particles or metallic particles of 1 .mu.m or more and have poor resistance to peeling at the sliding portions and poor film adhesion.
The chromium nitride film has insufficient wear resistance. Although the molybdenum nitride film is extremely hard with superior wear resistance, its brittleness may result in the generation of cracks or chips on the film due to repeated excessive stress from sliding movement, so that the cracks or chips may eventually lead to peeling.